Eddy current testing method

ABSTRACT

The eddy current testing apparatus includes a probe having an eddy current testing sensor including a pair of eddy current testing coils. The apparatus also includes an eddy current testing flaw detector inputting detection signals from the eddy current testing sensor. The diameter of a magnetic core used in each of the pair of eddy current testing coils is within the range of 0.1 mm to 0.5 mm.

CROSS-REFERENCES

This is a divisional of U.S. Ser. No. 11/862,897, filed Sep. 27, 2007,the content of which is hereby incorporated by reference.

CLAIM OF PRIORITY

The present application claims priority from Japanese application serialno. 2006-267571, filed on Sep. 29, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to an eddy current testing apparatus andan eddy current testing method, and more particularly, to an eddycurrent testing apparatus and an eddy current testing method preferablyapplicable to inspect internal surfaces of fork pin holes formed inblade forks for turbine blades attached to the rotor of a steam turbine.

To facilitate the manufacturing and maintenance of a steam turbine usedin a power generating plant, its rotational axis and turbine blades aremanufactured separately and then assembled. Specifically, blade forkportions formed at the roots of the turbine blades are assembled to discfork portions formed on a disc provided on the rotational axis of thesteam turbine, and the turbine blades are fixed to the disc by insertingpins into fork pin holes formed in these fork portions (see FIG. 2 inJapanese Patent Laid-open No. 2001-12208). As the steam turbine rotates,stress is applied to the structural material of the turbine blade in thevicinity of the fork pin holes in the blade fork portion. Accordingly,cracks may be generated in the vicinity of the fork pin holes in theturbine blade.

Conventional inspection for cracks in the vicinity of the fork pin holesin the turbine blade is performed by removing the turbine blade from thedisc and then applying magnetic particle testing (hereinafter referredto as MT). The MT is a method for detecting a leak of a magnetic fluxfrom a defect in a test object when a magnetic field is applied to thetest object. This MT is applied to the turbine blade, which is a testobject, as described below. While a magnetic field is applied to theblade fork portion of the turbine blade, the surface of the blade forkportion is coated with magnetic metal particles on which a fluorescentmaterial is applied, the fluorescent material being accumulated by themagnetic flux leaking from a defect. Ultraviolet rays are thenirradiated to the blade fork portion. The magnetic metal particlesaccumulate in, for example, a defect in a fork pin hole. Sinceultraviolet rays are irradiated, whether the magnetic metal particlesaccumulate can be determined by observing whether there is fluorescentlight, and thus whether there is a defect can be determined. Ininspection for defects on the basis of the MT, it is necessary not onlyto remove pins by which a disc fork portion of a disc and a blade forkportion of a turbine blade are joined but also to remove the turbineblade from the disc. When the turbine blade is inspected for defects onthe basis of the MT, therefore, it takes much time. Upon the completionof the inspection, an additional task is needed to fit the turbine bladeinto the disc and combine them with pins.

Eddy current testing (ECT) is often used as a method of inspecting thesurface of a test object for cracks. Particularly, the ECT is widelyused to inspect the internal surfaces of tubular objects such as heattransfer tubes because an ECT probe can be moved quickly in the tube andthus advantages of functions suitable for high-speed ECT inspection canbe fully taken. An example of ECT is described in Patent Laid-open No.Hei 8(1996)-145954. In the ECT disclosed in Japanese Patent Laid-openNo. Hei 8(1996)-145954, a test probe including an ECT sensor is insertedinto a tube to check whether the thickness of the tube is thinned andthe interior is corroded.

SUMMARY OF THE INVENTION

However, ECT has not been used for inspection for cracks on the internalsurfaces of fork pin holes in the steam turbine. As described later, theconventional ECT probe has not been sufficient to precisely detect theabove crack generated on the internal surface of a fork pin hole.

An object of the present invention is to provide an eddy current testingapparatus and eddy current testing method that can further improvetesting precision in eddy current testing.

A feature of the present invention for attaining the above object isthat the diameter of a magnetic core used as an eddy current testingcoil included in an eddy current testing sensor falls within the rangeof 0.1 mm to 0.5 mm. According to the present invention, in a structurein which blade fork portions formed at the roots of a plurality ofturbine blades disposed along the periphery of the disc of a turbinerotor are joined with disc fork portion formed on the disc by use ofpins, a crack which is generated on at least part of the internalsurface of a hole formed through the disc fork portion and blade forkportion by pulling out the pin can be precisely detected. Particularly,in the present invention, it is possible to detect a minute crack thatis generated near a juncture of two fork portions oppositely-disposed inadjacent blade fork portions, the juncture being formed on an internalsurface of a groove formed by the facing surfaces of these forkportions, the internal surface being at least part of the internalsurface of the hole.

To bring the diameter of the magnetic core within the range of 0.1 mm to0.5 mm is a new fining obtained by the inventors, as described later.

Another feature of the present invention is to insert a probe having aneddy current testing sensor into the hole, which is formed through thedisc fork portion and blade fork portion by pulling out the above pinwhile the blade fork portion is still inserted into the disc forkportion, and to perform eddy current testing for at least part of theinternal surface of the hole. This feature can extremely shorten a timetaken for the eddy current testing because it is not necessary to removethe turbine blade from the disc.

According to the present invention, testing precision in eddy currenttesting can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing a state in which supportingrods of the eddy current testing apparatus shown in FIG. 2 are alignedto fork pin holes formed in a disc.

FIG. 2 is a structural diagram showing an eddy current testing apparatusaccording to a preferred embodiment of the present invention.

FIGS. 3A to 3C are a structural diagram showing a probe shown in FIG. 2;FIG. 3A is a side view of a probe having an ECT sensor; FIG. 3B is across sectional view of the probe at a position at which the ECT sensoris set; FIG. 3C is a perspective view of a pair of ECT coils included inthe ECT sensor.

FIG. 4 is an explanatory drawing showing junctures between a fork of ablade fork portion 4 a and a fork of a blade fork portion 4 b, the bladefork portion 4 a being adjacent to the blade fork portion 4 b in theperipheral direction of the disc, and also the scanning by the probe.

FIG. 5 is a flowchart showing a procedure executed in ECT inspectionusing the eddy current testing apparatus shown in FIG. 2.

FIG. 6 is an explanatory drawing showing exemplary ECT signals outputfrom the eddy current testing apparatus shown in FIG. 2.

FIG. 7 is an enlarged perspective view showing the blade fork portion ofa turbine blade under ECT inspection.

FIG. 8 is a structural diagram showing a combination portion of theblade fork portion and the disc fork portion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will be described below.

Embodiment 1

An eddy current testing apparatus, which is a preferred embodiment ofthe present invention, inspects a blade fork portion formed at the rootof a turbine blade provided on the rotor of a steam turbine and a discfork portion formed on a disc. Specifically, the eddy current testingapparatus checks whether there is a crack on the internal surfaces offork pin holes formed in a blade fork portion and a disc fork portion.

Before explaining the eddy current testing apparatus in presentembodiment, the structure of the rotor of the steam turbine will beschematically explained with reference to FIGS. 1, 7, and 8, the rotorhaving turbine blades and a disc being test objects. The rotor (notshown) in the steam turbine has turbine blades 3 at a plurality ofstages, and is rotatably mounted in a turbine casing (not shown). Therotor forms the disc 1 on the rotary axis thereof. Many turbine blades 3disposed along the periphery of the disc 1 are removably attached to thedisc 1. A plurality of turbine blades 3 included in a turbine bladearray in a single stage are mutually linked at their tops by a shroud 7.

How the disc 1 and turbine blades 3 are joined together will bespecifically explained. Disc fork portions 2 are formed along the outerperiphery of the disc 1, and blade fork portions 4 are formed at theroots of the turbine blades 3. The blade fork portion is a embeddingportion of the blade 3. The shape of the disc fork portion 2 is suchthat a plurality of disc grooves 22 are formed in parallel in the axialdirection of the rotor at fixed intervals. The disc fork portion 2 formsforks (fork projections) 23, each of which is between two adjacent discgrooves 22. For example, six forks 23 are formed in the disc forkportion 2 for each turbine blade 3. The blade fork portion 4 is statedby forming a plurality of fork grooves 24 at the root of the turbineblade 3 at fixed intervals. The blade fork portion 4 has forks (forkprojections) 25, each of which is between two adjacent fork grooves 24.For example, five forks 25 constitute a blade fork portion 4. The numberof forks 23 and the number of forks 25 vary with the turbine blades atdifferent stages.

In each fork 23 of the disc 1, many fork pin holes (hereinafter simplyreferred to as pin holes) 5B are formed. The pin hole 5B is a roundpenetrated hole. Pin holes 5B are formed in each fork 23 for each of theturbine blades 3 disposed along the periphery of the disc 1 at fixedintervals. A plurality of pin holes 5B are also formed in a radialdirection of the disc 1 at equal intervals. In present embodiment, threepin holes 5B are formed in a radial direction for each turbine blade 3.The pin holes 5B formed in the radial directions are concentric.

Of the forks 25 of the blade fork portion 4 disposed on single turbineblade 3, three forks located at the center are denoted forks 25B. Theseforks 25B each have a plurality of pin holes 5A, which are roundpenetrated holes. In present embodiment, each fork 25B has three pinholes 5A at equal intervals in the longitudinal direction of the fork25B. The positions of the three pin holes 5A in the longitudinaldirection match the positions of the three pin holes 5B formed in theforks 23 in the radial direction. Of the forks 25 of the blade forkportion 4, forks at both ends of the blade fork portion 4 are denotedforks 25A. These forks 25A each have a plurality of grooves 5C whichhave a shape resulting from cutting the pin hole 5A in half (the grooveis referred to below as the half-round groove) and which are positionedat both ends facing in the peripheral direction of the disc 1. Threehalf-round grooves 5C are formed at the both ends each in thelongitudinal direction of the fork 25A. The position of the half-roundgroove 5C at one end matches the position of the pin hole 5A formed inthe fork 25B in the longitudinal direction of the fork 25. The positionof the half-round groove 5C at the other end matches the position of thepin hole 5A formed in the fork 25B of another turbine blade 3 adjacentto the turbine blade 3 in the peripheral direction of the disc 1. Thepin hole 5A, pin hole 5B, and half-round groove 5C have the same radius.

Each fork 25 of the turbine blade 3 is inserted into the disc grooves 22respectively. In other words, each fork 23 of the disc 1 is insertedinto a fork groove 24 of the turbine blade 3. In this state, a pluralityof turbine blades 3 are lined up on the periphery of the disc 1 in theperipheral direction. The turbine blades 3 disposed like this areattached to the disc 1 by a plurality of fork pins 6. These fork pins 6are inserted into the pin holes 5B formed in the forks 23, which arealigned in the axial direction of the rotor, the pin holes 5A formed inthe forks 25B, and the half-round grooves 5C (see FIG. 8). Three forkpins 6 are inserted into each fork 25 of a single turbine blade 3. Thesefork pins 6 are an outer fork pin 6A, a middle fork pin 6B, and an innerfork pin 6C. The three fork pins 6 are disposed in a radial direction ofthe disc 1; the outer fork pin 6A is at the outermost position, theinner fork pin 6C is at the innermost position, and the middle fork pin6B is at the middle position.

Suppose that a plurality of turbine blades 3 are disposed on the disc 1in the peripheral direction, as indicated by turbine blades 3 a, 3 b,and 3 c shown in FIG. 1. One pin hole similar to the pin hole 5A isformed by half-round grooves 5C that are formed in theoppositely-disposed end surfaces of the forks 25A of the adjacentturbine blades 3 a and 3 b, the pin hole extending across these forks25A (see FIG. 4). Junctures X and Y are formed between theoppositely-disposed forks 25A. The juncture Y is diametrically oppositeto the juncture X.

The eddy current testing apparatus in a preferred embodiment of thepresent invention will be described below in detail with reference toFIGS. 1 to 3. As shown in FIG. 2, the eddy current testing apparatus 19in present embodiment has a sensor unit 16, an eddy current flawdetector 17, and a computer 18. The sensor unit 16 includes a probe 8, apair of supporting rods 9, a flexible shaft portion 10, a supportingmember 13, and a positioning device 14. The pair of supporting rods 9are fixed to the supporting member 13. A linking member 36 links ends ofthe supporting rods 9. The pair of supporting rods 9 pass through acasing 10A of the flexible shaft portion 10. The flexible shaft portion10 is movable along the supporting rods 9 in the axial direction of thesupporting rods 9. The probe 8 is attached to the casing 10A anddisposed between the pair of supporting rods 9. The ends of the probe 8and supporting rods 9 are streamlined so that they can be insertedsmoothly into pin holes. The outer diameters of the probe 8 andsupporting rods 9 are the same as the inner diameters of the pin holes5A and 5B. The end of the supporting rods 9 and the probe 8 may bereplaceable according to the inner diameter of the pin holes formed inthe disc 1 etc. The positioning device 14 is omitted in FIG. 1.

The probe 8 has an ECT sensor 12 is mounted in a main probe body 8 a, asshown in FIGS. 3A and 3B. The ECT sensor 12 including ECT coils 12 a and12 b is disposed at the end portion of the main probe body 8 a. The ECTcoils 12 a and 12 b are disposed side by side in the main probe body 8a, from the outer surface of the main probe body 8 a toward the axialcenter of the main probe body 8 a. A space is left between the ECT coils12 a and 12 b in the peripheral direction of the main probe body 8 a(see FIG. 3B). The ECT coils 12 a and 12 b are each structured byforming a coil 29 around a magnetic core 15 (see FIG. 3C). The magneticcore 15 is made of a magnetic material such as ferrite. Both the ECTcoils 12 a and 12 b use a magnetic core 15 with diameter of 0.5 mm. TheECT sensor 12 is a self-induced ECT sensor based on a differential coilmethod. Both the ECT coils 12 a and 12 b use current to performexcitation and output detection signals. A feature of the self-inducedECT sensor based on the differential coil method in present embodimentis that when a metal member being the test object is made of a magneticmaterial and produces magnetic noise, the effect by the noise can becanceled with ease by detecting a difference between the signals fromthe two ECT coils 12 a and 12 b. When the effect by the noise is small,a cross correlation method, in which one of the two ECT coils 12 a and12 b is used for excitation and the other is used for detection, or aself-inducing method, in which a single coil used, may also be applied.

In addition to the casing 10A, the flexible shaft portion 10 has ahandle 11 rotatably attached to the casing 10A and a rotational forcetransmitting mechanism (not shown) disposed in the casing 10A. Therotational force transmitting mechanism has a rotational axis and abevel gear and the like for transmitting the rotational force of thehandle 11 to the probe 8. When the handle 11 is rotated, the probe 8rotates. An angle meter (not shown) is attached at another end, which isin the casing 10A, of the main probe body 8 a. The angle meter detectsthe rotational angle of the probe 8. Another handle and transmissionmechanism (not shown) may be disposed in the supporting member 13 sothat the pair of supporting rods 9 can be moved right and left withrespect to the probe 8. When the other handle is rotated, the supportingrods are moved right or left and thereby the space between thesupporting rods 9 can be adjusted according to the space between the pinholes 5B, as described later. To allow the pair of supporting rods 9 tomove right and left, the penetrated holes formed in the casing 10Athrough which the supporting rods 9 pass need to be elongated to theright and left.

The positioning device 14 has a plate-like position setting member 32and a stopper member 34. One end of the position setting member 32 isattached to the supporting member 13, and the other end is attached toone of the supporting rods 9. The position setting member 32 is disposedin parallel to the supporting rods 9 and outside the flexible shaftportion 10. When the flexible shaft 10 moves in the axial direction ofthe supporting rods 9, the movement is not restricted by the positionsetting member 32. A plurality of positioning holes 33 are formed in theposition setting member 32 according to the positional relationshipamong the pin holes 5B, half-round grooves 5C, and pin holes 5A. Thestopper member 34 is disposed on a side of the casing 10A and extendstoward the position setting member 32. The stopper member 34 has astopper portion 35 that can be inserted into and removed from thepositioning hole 33. The stopper part 35 is structured in such a waythat, for example, a metal ball movable in a cylindrical member in itsaxial direction is pressed downward by a coil spring. The metal balldoes not come off the cylindrical member.

A power supply (not shown) for supplying excitation current and thecoils 29 of the ECT coils 12 a and 12 b are connected with differentwires. These wires are each provided with a switch (not shown) disposedin the casing 10A. A first switch is connected to the coil 29 of the ECTcoil 12 a, and a second switch is connected to the coil 29 of the ECTcoil 12 b.

A multi-conductor cable 21 is connected to the eddy current flawdetector 17 and extends up to the inside of the casing 10A of theflexible shaft portion 10. For example, a conductor included in themulti-conductor cable 21 is a signal line that is connected to the coil29 of the ECT coil 12 a and transmits a signal detected by the ECT coil12 a. Another conductor is a signal line that is connected to the coil29 of the ECT coil 12 b and transmits a signal detected by the ECT coil12 b. Another conductor transmits a switching command output from theeddy current flaw detector 17 to the first and second switches. Anotherconductor is connected to the above-mentioned angle meter. The eddycurrent flaw detector 17 is connected to the computer 18 through a cable28. The computer 18 has a function for outputting control commands, suchas an inspection start command and inspection termination commanddescribed later, and also has a signal processing function forprocessing ECT signals, which are output from the eddy current flawdetector 17 according to the signals detected by the ECT coils 12 a and12 b. Accordingly, the computer 18 practically includes a controller foroutputting control commands and a signal processor for processing itsECT signals.

As described above, the eddy current testing apparatus 19 is used toperform ECT inspection of the disc fork portion 2 of the rotor of thesteam turbine and the blade fork portion 4. The ECT inspection will bedescribed below in detail according to the flowchart, shown in FIG. 5,which comprises steps 40 to 45. First, the turbine casing isdisassembled, and the rotor, on which a plurality of turbine blades 3are disposed, is taken out from the turbine casing. The taken out rotoris rotatably placed on a pair of supporting bases mounted on the floor.Fork pins 6 are then pulled out of the disc 1 (step 40). In the processof removing the fork pins 6, the outer fork pins 6A, which are disposedat the outermost positions for a turbine blade array in a single stage,are all pulled out. Although all turbine blades 3 included in theturbine blade array face downward, they do not come off because they arejoined with the disc 1 by the middle fork pins 6B and inner fork pins6C.

The sensor unit 16 is mounted in an inspection place (step 41). Thesupporting member 13 of the sensor unit 16 is mounted on a supportingdevice (not shown) placed on the floor so as to be slidable in the axialdirection of the supporting rods 9. The height of the supporting devicecan be adjusted according to the position of the pin hole underinspection. The targets that undergo ECT inspection are the internalsurfaces of pin holes 5A and 5B and half-round grooves 5C into which anouter fork pin 6A has been inserted. Specifically, ECT inspection isperformed for the internal surface of a pin hole 5B into which a singleouter fork pin 6A has been inserted as well as the internal surfaces ofa pin hole 5A and half-round grooves 5C on a single turbine blade 3.Upon the completion of the inspection, ECT inspection is performed forthe internal surface of a pin hole 5B into which another outer fork pin6A has been inserted, which is adjacent to the outer fork pin 6A in theperipheral direction of the disc 1 as well as the internal surfaces of apin hole 5A and half-round grooves 5C on another turbine blade 3. Forexample, suppose that the internal surfaces of first pin holes under ECTinspection are the internal surface of the pin hole 5A, formed in theblade fork portion 4 b of the turbine blade 3 b, into which the outerfork pin hole 6A has been inserted, as well as the internal surfaces ofthe pin hole 5B and half-round grooves 5C on an extending line of theaxial center of the pin hole 5A. The pin hole 5A, pin hole 5B, andhalf-round grooves 5C are collectively referred to below as pin holesunder inspection. The probe 8 is inserted into the pin hole underinspection, as described later. The pair of supporting rods 9 areinserted into the pin holes 5A, into which the outer fork pin 6A hasbeen inserted, formed in the blade fork portions 4 a and 4 c of theturbine blades 3 a and 3 c, each of which is adjacent to the turbineblade 3 b in the peripheral direction, as well as into the pin holes 5Band half-round grooves 5C on extending lines of the axial center ofthese pin holes 5A. These pin holes 5A and 5B and the half-round grooves5C, into which the supporting rods 9 are inserted, are collectivelyreferred to below as supporting pin holes. In FIG. 1, the end portionsof the pair of supporting rods 9 are aligned to two supporting holes,corresponding to the turbine blades 3 a and 3 c, into which the endportions of the supporting rods 9 will be inserted. When an operatormanually slides the supporting member 13 with respect to the supportingdevice, the pair of supporting rods 9 are inserted into thecorresponding supporting pin holes. When the pair of supporting rods 9are inserted into the corresponding supporting pin holes, the sensorunit 16 is set at the inspection place. While the ECT inspection is inprogress, the supporting rods 9 are held by the supporting member 13mounted on the supporting device and the supporting pin holes. It isalso possible to use an arrangement in which a motor is mounted in thesupporting device so as to move the supporting member 13 toward the discfork portion 2 by driving the motor.

Next, the probe 8 is inserted into the pin holes under inspection (step42). Specifically, the operator manually moves the flexible shaftportion 10 toward the disc fork portion 2 by using the supporting rods 9as a guide. Due to this movement, the end portion of the probe 8 isinserted into the pin holes, including the pin hole 5A formed in theblade fork portion 4 of the turbine blade 3 b. The positioning device 14positions the ECT sensor 12 attached to the probe 8 in the pin holesunder inspection. Specifically, the positioning is performed byinserting the stopper portion 35 of the stopper member 34 into thepositioning hole 33 formed in the position setting member 32 at aprescribed position. When the flexible shaft portion 10 is moved, themetal ball pushes the coil spring upward, so the stopper portion 35easily comes off the positioning hole 33, making the flexible shaftportion 10 movable. The ECT sensor 12 is then positioned at a prescribedposition, for example, an angular portion formed by two facinghalf-around grooves 5C formed in the forks 25A of the adjacent turbineblades 3 b and 3 a.

The ECT sensor 12 is zero-adjusted at a point on other than thejunctures of forks of adjacent blade fork portions (step 43). A signaldetected by the angle meter is input to the eddy current flaw detector17 through the cable 21 and then input to the computer 18 through thecable 28. The computer 18, that is, the signal processing unit, outputsinformation about the rotational angle of the probe 8, which is detectedby the angle meter, to a display unit 20. The operator determineswhether the ECT sensor 12 faces a normal portion, that is, other thanthe junctures X and Y, with reference to the displayed information aboutthe rotational angle. If the ECT sensor 12 faces one (X or Y) of thejunctures, the operator rotates the handle 11 to turn the probe 8 sothat the ECT sensor 12 faces a portion (preferably, a portion free fromcracks) on the internal surfaces of the half-round grooves 5C betweenthe junctures X and Y. In this state, the operator enters an inspectionstart signal from an input device (for example, a keyboard) connected tothe controller. The controller (computer 18) outputs an inspection startcommand to the eddy current flaw detector 17 in response to theinspection start signal. Upon the input of the inspection start command,the eddy current flaw detector 17 outputs a start command to the firstswitch and second switch to close them. Excitation current is suppliedfrom a power supply to the coils 29 of the ECT coils 12 a and 12 b. Eddycurrent is then generated on the surface of the half-round grove 5C, andcurrents inducted by the eddy current are generated in the coils 29. Thecurrents generated in the coils 29 become signals detected by the ECTcoils 12 a and 12 b and are input to the eddy current flaw detector 17through the cable 21. The eddy current flaw detector 17 takes adifference between these detection signals by a circuit providedtherein, and outputs the resulting ECT signal to the signal processingunit (computer 18). The ECT signal is output from the signal processingunit to the display unit 20 and displayed on the display unit 20. If theECT signal is 0, it indicates that the ECT sensor 12 is zero-adjusted.If the ECT signal is not 0, the balance of a bridge circuit (or anequivalent circuit) provided in the eddy current flaw detector 17 isadjusted so that the ECT signal becomes 0. This completes the zeroadjustment of the ECT sensor 12.

Upon the completion of the zero adjustment, the probe undergoes rotaryscanning (step 44). Specifically, the operator operates the handle 11 torotate the probe 8 one turn in the pin hole under inspection. In thisrotary operation, the ECT sensor 12 rotates along the internal surfacesof the pair of half-round grooves 5C. ECT inspection is performed forthe internal surfaces of the pair of half-round grooves 5C, which areincluded in the pin holes under inspection and face each other. Sinceexciting current is supplied to the ECT coils 12 a and 12 b, a detectionsignal output from the ECT coil 12 a and another detection signal outputfrom the ECT coil 12 b are both input to the eddy current flaw detector17, as described above. The eddy current flaw detector 17 outputs an ECTsignal, which is a differential signal obtained according to both thesedetection signals. The ECT signal is input to the signal processingunit, in which the signal is processed. The signal processing unitcreates image information that represents the relation between theamplitude of the ECT signal and the rotational angle by using, forexample, the input ECT signal and the information about the rotationalangle of the probe 8, which is input from the angle meter. This imageinformation includes, for example, image information showing therelation between the X-direction component of the amplitude and therotational angle and another image information showing the relationbetween the Y-direction component of the amplitude and the rotationalangle. The signal processing unit outputs these image information to thedisplay unit 20, and the display unit 20 displays these imageinformation. When the above angle meter is not attached to the sensorunit 16, the signal processing unit creates image information thatrepresents the relation between the amplitude of the ECT signal andtime.

FIGS. 6A and 6B show exemplary ECT signals output from the eddy currentflaw detector 17 to which signals detected by the ECT coils 12 a and 12b are input in ECT inspection of the internal surfaces of the pair ofhalf-round grooves 5C described above. Even in a normal state in whichthe internal surfaces of the pair of half-round grooves 5C are free fromcracks, ECT pulses C1 with a large amplitude are generated at thejunctures X and Y, in response to the shapes of the mating planes at thejunctures X and Y (see FIG. 6A). At positions other than the junctures Xand Y, the ECT signal is zero. When the internal surface of a half-roundgroove 5C has cracks 27, near the junctures, that extend the axialdirection of the pin hole under inspection (see FIG. 7), the ECT signaloutput from the eddy current flaw detector 17 is, for example, as shownin FIG. 6B. In addition to the ECT pulses C1 generated at the X and Yjunctures, the ECT signal includes ECT pulses with an amplitude largerthan zero, the ECT pulses being generated at the positions at which thecracks are present. When the internal surface of a half-round groove 5Con the blade fork portion 4 a includes a crack 27 near the juncture X,the ECT signal includes an ECT pulse C2 with an amplitude larger thanzero at the position at which the crack 27 is present. When the internalsurface of a half-round groove 5C on the blade fork portion 4 b includesa crack 27 near the juncture Y, the ECT signal includes an ECT pulse C3with an amplitude larger than zero. The cracks 27 generated on theinternal surfaces of the half-round grooves 5C are small, so theamplitudes of the ECT pulses C2 and C3 are extremely smaller than theamplitude of the ECT pulse C1.

When a crack 27 is generated near the juncture X or Y, the conventionalECT sensor is affected by the ECT pulse C1 generated at the juncture, sothe ECT pulse corresponding to an extremely small crack 27 cannot beseparated from the ECT pulse generated at a juncture. Therefore, theconventional ECT sensor cannot detect a crack 27 generated near thejuncture. The inventors examined various methods for detecting a crack27 generated on the internal surface of a half-round groove 5C near itsjuncture, without being affected by the ECT pulse C1. The inventors thennewly found through experiments that when the diameter of the magneticcore of the ECT sensor is 0.5 mm or less, the spatial resolution of theECT sensor can be improved and thereby an ECT pulse generated on theinternal surface of the half-round groove 5C near its juncture inresponse to the crack 27 can be separated from the ECT pulse at thejuncture. However, when the diameter of the magnetic core is less than0.1 mm, fabrication of the ECT coils become difficult. When the diameterof the magnetic core of the ECT sensor falls within the range of 0.1 mmto 0.5 mm, a crack 27 generated on the internal surface of a half-roundgroove 5C near its juncture can be detected.

Since the diameter of the magnetic core 15 is 0.5 mm in presentembodiment, a crack 27 generated on the internal surface of a half-roundgroove 5C near its X or Y juncture can be detected.

Upon the completion of the ECT inspection for the above pair ofhalf-round grooves 5C, the operator manually moves the flexible shaftportion 10 to move the probe 8 within the above pin holes underinspection, which corresponds to the blade fork portion 4 b, so that theECT sensor 12 is positioned at a position in other pin holes 5B and 5Ain succession. The probe 8 is rotated at each position, and ECTinspection is performed at that position. Detection signals output fromthe ECT sensor 12 in the ECT inspection are input to the eddy currentflaw detector 17 and output as ECT signals. The ECT signals are input tothe signal processing unit and processed therein.

Whether there is a crack is determined (step 45). Specifically, theoperator determines whether there is a crack on the internal surfaces ofthe half-round grooves 5C that have undergone ECT inspection, based onthe information, which is displayed on the display unit 20, obtained inthe ECT signal processing by the signal processing unit.

When the internal surface of any of the pin holes 5A and 5B andhalf-round grooves 5C, which are pin holes under inspection, does nothave a crack, the flexible shaft portion 10 is manually moved to pullout the probe 8 from the pin holes under inspection. The supportingmember 13 is also manually moved to pull out the pair of supporting rods9 from the supporting pin holes. The next pin holes under inspection arethe pin holes 5A and 5B and half-round grooves 5C into which an outerfork pin 6A, by which the turbine blade 3 c adjacent to the turbineblade 3 b is joined with the disc 1, has been inserted. The position ofthe sensor unit 16 is adjusted in the height direction so that the probe8 faces the front of the next pin holes under inspection. Specifically,as described above, the supporting rods 9 are inserted into thesupporting pin holes adjacent to this pin hole under inspection, and theprobe 8 is then inserted into the pin holes under inspection. ECTinspection is performed for the internal surfaces of the pin holes andlike under inspection. As described above, ECT inspection is performedfor all pin holes under inspection into which all outer fork pins 6Ahave been inserted, the outer fork pins joining a turbine blade array ina single stage to the disc 1.

If a crack is found in a pin hole under inspection into which an outerfork pin 6A has been inserted, for example, in half-round grooves 5C aspart of the pin holes under inspection including the outermost pin hole5A in the blade fork portion 4 b, the middle fork pin 6B for the turbineblade 3 b is pulled out of the disc 1. Of the pin holes under inspectionincluding the pin holes in which the middle fork pin 6B has beeninserted, ECT inspection is performed, as described above, for theinternal surfaces of the half-round grooves 5C inside the half-roundgroove 5C in which the crack has been found. If a crack 27 is also foundin these half-round grooves 5C, the inner fork pin 6C disposed on theinner side is also pulled out. ECT inspection is performed for theinternal surface of the half-round grooves 5C as part of the pin holesunder inspection into which the middle inner fork pin 6C has beeninserted. The middle fork pin 6B and inner fork pin 6C are inserted intothe appropriate pin holes after the ECT inspection for theircorresponding pin holes under inspection has been completed.Accordingly, if height adjustment by the supporting device is notpossible, even when the rotor is rotated on a pair of supporting tablesfor alignment with the probe 8 before ECT inspection is performed forpin holes under inspection corresponding to another outer fork pin 6A,the inserted middle fork pin 6B and inner fork pin 6C prevent theturbine blade 3 b from coming off.

After ECT inspection is completed for all pin holes under inspectioncorresponding to all the above outer fork pins 6A, new outer fork pins6A, which are prepared separately, are inserted into appropriate pinholes formed in, for example, the disc 1, etc.

Of the pin holes 5A and 5B and half-round grooves 5C formed in each fork25, which are included as pin holes under inspection, ECT inspection maybe performed only for the internal surfaces of a pair of half-roundgrooves 5C that are located at the frontmost or backmost position in thedirection of the insertion of the probe 8.

The present embodiment can improve detection accuracy for cracks becausethe ECT sensor 12 having the magnetic core 15 with a diameter of 0.5 mmis used. Particularly, it is possible to detect even a small crack 27generated on internal surfaces of half-round grooves 5C near a junctureX or Y of forks 25A of adjacent blade fork portions 4 a and 4 b. Inpresent embodiment, since the ECT coils 12 a and 12 b of the ECT sensor12 are disposed so that their axial centers face the outer surface ofthe main probe body 8 a, it is possible to easily detect a crack, on theinternal surface of a pin hole 5A or 5B or a half-round groove 5C, thatextends in the axial direction of the pin hole. Since the probe 8 isrotated, the entire inner surface of a pin hole or the like can bechecked for a crack.

In present embodiment, the rotational angle of the probe 8 is alsodetected. When both the rotational angle and the ECT signal areconsidered, the operator can recognize which of the adjacent turbineblades 3 a and 3 b has a crack 27 on the internal surface of thehalf-round groove 5C formed on the blade fork portion 4, as illustratedin FIG. 6B. Furthermore, the operator can not only recognize thepresence of the above crack 27 but also know its position in theperipheral direction by displaying rotational angle information and theECT signals.

In ECT inspection, if the distance (referred to below as the liftoff)between the ECT sensor and the surface of the target under inspectionchanges, a signal detected and output also changes. In precisemeasurement of cracks, therefore, the change caused by a liftoff in thedetection signal needs to be eliminated. To achieve this, the probe 8must be firmly supported. However, the area around a disc fork portion2, which undergoes ECT inspection in present embodiment, is narrow, soto find a way for supporting the probe 8 was a problem that is difficultto achieve. The inventors were insistent in studying solutions foreliminating liftoff factors when ECT inspection was performed for theinternal surfaces of the pin holes under inspection in the disc forkportion 2 and blade fork portion 4. The inventors then arrived at a newidea that pin holes formed in the disc 1 etc. so as to accept fork pins6 are used to hold the supporting member that supports the probe 8. Apair of supporting rods 9 were thus provided to the sensor unit 16 andinserted into two supporting pin holes formed in the disc 1 etc., nearthe pin hole under inspection (preferably, the supporting pin holesadjacent to the pin hole under inspection) so that the probe 8 is heldby the pair of the supporting rods 9. Since, in present embodiment, theprobe 8 is held by the supporting rods 9 that are inserted intosupporting pin holes formed in the disc 1, etc. to accept fork pins 6,as described above, the probe 8 is firmly held with high precision.Accordingly, changes in liftoff can be substantially reduced and therebyliftoff-caused changes in detection signals output from the ECT coils 12a and 12 b can be substantially reduced, improving crack detectionprecision. The supporting rods 9 function as a holding member and guidemember for the probe 8 attached to the flexible shaft portion 10. It isalso possible to support the probe 8 by inserting a single supportingrod into a supporting pin hole. However, when two supporting rods 9 areinserted into two supporting pin holes, as in this embodiment, the probe8 is supported more firmly and thereby changes in liftoff can be madesmaller.

In present embodiment, it suffices to remove only fork pins 6 that arepresent in an area to be subject to ECT inspection from the disc 1,etc., so the ECT inspection can be performed for the internal surface ofa pin hole under inspection with the turbine blade 3 attached to thedisc 1. This eliminates the need to remove the turbine blade 3 from thedisc 1 when ECT inspection is performed for the fork portions,substantially shortening a time taken for ECT inspection to be performedfor the disc fork 2 and blade fork 4. Furthermore, since ECT inspectionis performed for the internal surface of a pin hole under inspectionwith the turbine blade 3 attached to the disc 1, ECT inspection can beperformed for the internal surfaces of the pin holes formed in the discfork 2 and blade fork 4 and the internal surfaces of the half-roundgrooves 5C formed in the blade fork 4 in succession with the probe 8left inserted into the pin hole under inspection. This scheme alsocontributes to further shortening in ECT inspection time. One factor ofshortening of the above ECT inspection time is an arrangement in whichthe supporting rods 9 are inserted into supporting pin holes.

In present embodiment, the outer fork pin 6A, disposed at the outermostposition in a radial direction of the disc 1, is also pulled out, andECT inspection is performed for the pin hole under inspection into whichthe outer fork pin 6A has been inserted. One turbine blade 3 is joinedto the disc 1 by a middle pin 6B and an inner pin 6C, which are disposedin a radial direction of the disc 1. Due to this arrangement, even whenall outer fork pins 6A disposed in a radial direction of the disc 1 arepulled out of a turbine blade array in a single stage during ECTinspection, the turbine blades 3 do not fall off the disc 1. In presentembodiment, ECT inspection is first performed for the pin hole underinspection into which an outer fork pin 6A has been inserted. If a crackis found on the internal surface of the pin hole under inspection, ECTinspection is performed for the pin hole under inspection into which themiddle fork pin 6B has been inserted, the middle fork pin 6B beingpositioned inside the outer fork pin 6A. In this embodiment, in whichECT inspection is performed as described above, the number of fork pins6 pulled out of the disc 1 can be substantially reduced, so time takenin ECT inspection can be further shortened. The number of fork pins 6 tobe attached to the disc 1 after the completion of the ECT inspection isalso reduced, so working time taken to attach the fork pins 6 is alsogreatly shortened. While the steam turbine is operated, stress isgenerated in areas, in the fork, around the pin holes under inspectioninto which the fork pin 6 has been inserted. The stress in the areacorresponding to the outer fork pin 6A is greater than the stress in theareas corresponding to the middle fork pin 6B and inner fork pin 6C.Accordingly, if there is no crack on the internal surface of the pinhole under inspection into which the outer fork pin 6A has beeninserted, no cracks are generated on the internal surfaces of the pinholes under inspection into which the middle fork pins 6B and inner 6Chave been inserted.

In present embodiment, of a plurality of fork pins 6 attached to thedisc 1, all outer fork pins 6A attached at outermost positions areremoved and ECT inspection is performed. It is also possible to removeall outer fork pins 6A, middle fork pins 6B, and inner fork pins 6Cattached to the disc 1 within the range of, for example, a 120° areaextending upward from the axial center of the disc 1 and then performECT inspection by using the eddy current testing apparatus 19 for allthe pin holes under inspection into which these fork pins have beeninserted. Upon the completion of the ECT inspection in this range, theremoved fork pins 6 are attached to the disc 1, and the rotor is rotatedso that another 120° area faces upward. All fork pins 6 are pulled outof the disc 1 in the other 120° area and ECT inspection is thenperformed. This ECT inspection is repeatedly performed for each 120°area.

In the above ECT inspection as well, the turbine blades 3 do not falloff the disc 1, eliminating the need to remove the turbine blades 3 fromthe disc 1. That is, when ECT inspection is performed by using the probe8 for the internal surfaces of pin holes under inspection, the bladefork portions 4 can be left inserted into the disc fork portion 2. ThisECT inspection takes a longer inspection time than in the embodimentdescribed earlier, but takes a shorter inspection time than theconventional inspection in which the turbine blades 3 are removed fromthe disc 1 to check the blade fork portions for cracks.

1. An eddy current testing method for a turbine rotor comprising a disc,a plurality of turbine blades disposed along the periphery of said disc,and a plurality of pins for joining a blade fork portion formed on eachof said plurality of turbine blades to a disc fork portion formed onsaid disc, comprising the steps of: inserting a probe having an eddycurrent testing sensor into a first hole formed through said disc forkportion and a first blade fork portion of a first turbine blade of saidplurality of turbine blades by pulling out a first pin, which joins saidfirst turbine blade to said disc, of said plurality of pins; inserting asupporting member for supporting said probe into a second hole formedthrough said disc fork and a second blade fork portion of a secondturbine blade, which is different from said first turbine blade and ispositioned in said periphery, of said plurality of turbine blades bypulling a second pin, which joins said second turbine blade to saiddisc, of said plurality of pins; and performing eddy current testing forat least part of the internal surface of said first hole by using saidprobe.
 2. The eddy current testing method according to claim 1, whereinsaid first pin being pulled out is disposed at the outermost position ofsaid plurality of pins disposed in a radial direction of said disc andjoining said first blade fork portion to said disc fork portion.
 3. Theeddy current testing method according to claim 1, wherein said secondpin being pulled out is disposed at the outermost position of saidplurality of pins disposed in a radial direction of said disc andjoining said second blade fork portion to said disc fork portion.
 4. Theeddy current testing method according to claim 1, wherein said probe isinserted into said first hole by guiding a probe holding memberattaching said probe along said supporting member.
 5. The eddy currenttesting method according to claim 1, wherein said supporting member isinserted into said second hole, and after insertion of said supportingmember, said probe is inserted into said first hole.
 6. The eddy currenttesting method according to claim 1, wherein another supporting memberfor supporting said probe is inserted into a third hole formed throughsaid disc fork portion and a third blade fork portion of a third turbineblade of said plurality of turbine blades by pulling out a third pin,which is positioned in said periphery and joins said third turbine bladeto said disc, of said plurality of pins in a state that said firstturbine blade is placed between said second turbine blade and said thirdturbine blade.
 7. The eddy current testing method according to claim 6,wherein said third pin being pulled out is disposed at the outermostposition of said plurality of pins disposed in a radial direction ofsaid disc and joining said third blade fork portion to said disc forkportion.
 8. An eddy current testing method for a turbine rotorcomprising a disc, a plurality of turbine blades disposed along theperiphery of said disc, and a plurality of pins for joining a blade forkportion formed on each of said plurality of turbine blades to a discfork portion formed on said disc, comprising the steps of: pulling afirst pin, which joins a first turbine blade of said plurality ofturbine blades to said disc, of said plurality of pins out of a firstblade fork portion of said first turbine blade; pulling a second pin,which joins a second turbine blade of said plurality of turbine bladesto said disc, of the plurality of pins out of said first blade forkportion a second blade fork portion of said second turbine blade;inserting a probe having an eddy current testing sensor into a firsthole formed through said disc fork portion and said first blade forkportion by pulling out said first pin; inserting a supporting member forsupporting said probe into a second hole formed through said disc forkand said second blade fork portion by pulling out said second pin; andperforming eddy current testing for at least part of the internalsurface of said first hole by using said probe, wherein said secondturbine blade is different from the first turbine blade.
 9. The eddycurrent testing method according to claim 8, wherein said first pinbeing pulled out is disposed at the outermost position of said pluralityof pins disposed in a radial direction of said disc and joining saidfirst blade fork portion to said disc fork portion.
 10. The eddy currenttesting method according to claim 8, wherein said second pin beingpulled out is disposed at the outermost position of said plurality ofpins disposed in a radial direction of said disc and joining said secondblade fork portion to said disc fork portion.
 11. The eddy currenttesting method according to claim 8, wherein said probe is inserted intosaid first hole by guiding a probe holding member attaching said probealong said supporting member.
 12. The eddy current testing methodaccording to claim 8, wherein said supporting member is inserted intosaid second hole, and after insertion of said supporting member, saidprobe is inserted into said first hole.
 13. The eddy current testingmethod according to claim 8, wherein another supporting member forsupporting said probe is inserted into a third hole formed through saiddisc fork portion and a third blade fork portion of a third turbineblade of said plurality of turbine blades by pulling out a third pin,which is positioned in said periphery and joins said third turbine bladeto said disc, of said plurality of pins in a state that said firstturbine blade is placed between said second turbine blade and said thirdturbine blade.
 14. The eddy current testing method according to claim13, wherein said third pin being pulled out is disposed at the outermostposition of said plurality of pins disposed in a radial direction ofsaid disc and joining said third blade fork portion to said disc forkportion.